1. Field of the Invention
The present invention pertains to an improved oscillator circuit having increased signal power.
2. Description of the Related Art
Oscillator circuits include a tank section having one or more inductance and capacitance elements which cause signal oscillation. All tank circuits inherently exhibit losses that are caused, for example, by resistance in the inductance elements. Such losses result in attenuation or decay over time of an oscillated output signal. In each oscillator signal cycle, a maximum of the losses coincides with the maximum value of the oscillated output signal, i.e. the maximum amplitude of the oscillated signal. To combat this problem, an active stage is included in oscillator circuits to compensate for signal decay by adding energy to the tank. A problem with such an approach, however, is that the DC power consumed by the circuit is usually much greater than the signal power it produces. And, as discovered by the inventor recently, to produce more signal power from a given amount of DC power, it is necessary to replenish the energy into the tank in as narrow a pulse as possible.
A known and widely used oscillator circuit suffering from the drawbacks discussed above is the Colpitts oscillator 10 shown in FIG. 1 which provides single-ended oscillation especially when the losses in the tank are large or the quality factor is small. This oscillator includes a tank stage having an inductor L and a capacitor pair C.sub.1, C.sub.2, driven by a power replenishing stage having a transistor Q and a DC current source 12 which draws a constant current I. As shown, capacitors C.sub.1, C.sub.2 are connected at a node N.sub.1 which is located at the source terminal of the transistor Q where a voltage V.sub.1 is present. Capacitor C.sub.1 is connected to inductor L at a node N.sub.2, at which a voltage V.sub.2 is present. The power supply V, bias voltage V.sub.B and DC current source 12 provide proper bias condition for the transistor Q. Voltage V.sub.2 represents the oscillated output signal of circuit 10.
When the tank stage begins to oscillate, the value in signal V.sub.2 increases and decreases periodically around its quiescent point V. The signal V.sub.1 is basically a voltage divided value of signal V.sub.2 between capacitor C.sub.1 and C.sub.2 and thus, also varies around its quiescent point. As signals V.sub.2 and V.sub.1 increase, the transistor Q--which has a turn-on or threshold voltage V.sub.TH --will be turned off because the voltage between the gate and source terminals (V.sub.gs) decreases below the value of V.sub.TH. As a result, and because capacitor C.sub.1 resembles an open-circuit condition for a DC current, the current I required by the current source 12 is provided through the discharge of capacitor C.sub.2. As oscillation continues, this causes the values of V.sub.1 and V.sub.2 to decrease until the voltage V.sub.gs exceeds V.sub.TH. At this point, transistor Q turns on to provide current to current source 12 and to recharge capacitor C.sub.2. Consequently, the signals V.sub.1 and V.sub.2 increase again.
A drawback of the prior art circuit 10 of FIG. 1 is that the conduction duration of the transistor is dictated by a single control signal namely, V.sub.1, which is related to the output signal V.sub.2 through the ratio of C.sub.1 and C.sub.2. The choice of C.sub.2 thus serves dual diametrically opposing functions. In particular, C.sub.2 needs to be small in order to produce a large variation in signal V.sub.1 to turn the transistor on and off in as short a duration as possible. On the other hand, C.sub.2 needs to be large in order to provide the current drawn by the current source 12 in as long a duration as possible. The limitation on the duration of transistor conduction resulting from this dilemma restricts the capabilities of the prior art circuit for attaining an increased signal power and, consequently, an increased frequency stability.